The invention relates generally to color television receivers, and more particularly to an electron beam deflection system for color cathode ray tubes having three guns located in-line in the tube neck, a wide deflection angle and a large phosphor screen.
Deflection yokes are commonly arranged on the cathode ray tube neck for the purpose of generating magnetic fields which act on the electron beams to cause the beams to scan the whole phosphor screen of the tube. The deflection yokes commonly have two windings wound on at least one ferrite core, one winding acting for horizontal deflection and the other acting for vertical deflection. Deflection windings are classified into two general types according to their form, namely, the saddle type and the toroidal type. The type chosen for use depends upon the design of the associated deflection circuits to which they are connected, each type having advantages and disadvantages corresponding to a particular application.
To achieve the deflection and the self-convergence of the electron beams over the total area of the phosphor screen, the deflection yoke must generate, at least in the section nearest to the screen, herein called front of the yoke, a non-uniform magnetic field which is pincushion-shaped for horizontal deflection, and which is barrel-shaped for vertical deflection.
In general, the center electron beam does not receive the same deflection force as the two side electron beams because the two side beams are nearer to the deflection coils than said center beam. Therefore, the center beam deflection is not the same as that of the side beams. This phenomenon is commonly called the coma effect. Obviously, the coma effect increases with the width of the phosphor screen, and becomes very noticable on, for example, a 26-inch phosphor screen. Correction of this effect can be achieved by several methods depending upon the type of cathode ray tube used. For example, when utilizing a small neck cathode ray tube, i.e. 28 mm diameter at the gun end, a magnetic shunt may be located inside the glass of the tube to decrease the force of the deflection fields on the side beams and thereby correct the coma effect. When utilizing a large neck cathode ray tube, i.e. 36 mm diameter at the gun end, a magnetic shunt may be located outside of the glass of the tube and behind the deflection yoke. With the foregoing modifications, classical toroidal deflection yokes have been used to achieve self-convergence without coma effect. But the foregoing solutions require relatively large volumes of space due to the added tube neck-components.
Another solution used in the prior art to achieve beam self-convergence without coma effect is modification of the magnetic field distribution between the front and the back of the deflection yoke, for instance by manufacturing a deflection yoke having two or more axially positioned ferrite cores on which the coils are separately wound, generating in the back of the yoke a magnetic field to compensate for coma effects, and in the front of the yoke a magnetic field for deflection and for self-convergence. However, this latter solution is relatively expensive.
Prior art solutions to the problem also includes the design of a deflection yoke having saddle-shaped coils with conductor distribution positioned on a ferrite core to provide a magnetic field shape for deflection and self-convergence in the front of the yoke and for correction of the coma effect in the back of the yoke. However, the required precise location for each turn of each coil slows production and otherwise increases costs.
A magnetic field having different distributions at the front and rear parts of the yoke may also be generated by using a ferrite core with a periphery taking the form of curves of varying radii of curvature. The use of such a ferrite core is also expensive.
Another method for modifying the magnetic field distribution is through use of a deflection yoke which has a plurality of separate windings, each winding having a like number of turns toroidally wound in a generally axial direction, and with a predetermined spaced relation about the periphery of the annular magnetic core. The individual windings may be interconnected to form three groups of windings, one group of which is connected to a horizontal deflection current source, one group to a vertical deflection current source and the third group to both the horizontal and vertical deflection current sources. Briefly, the toroidal yoke may have as many as 22 windings having an equal number of turns and connected to a supply bridge network. The design requires a complicated supply network for varying the magnetic field and is limited to use with a small phosphor screen where the coma effect correction is not required.